Typically, a solid oxide fuel cell (SOFC) employs a solid electrolyte of ion-conductive oxide such as stabilized zirconia. The solid electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, normally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
As the fuel gas supplied to the fuel cell, normally, a hydrogen gas produced from hydrocarbon raw material by a reformer is used. In general, in the reformer, a reforming raw gas is obtained from a hydrocarbon raw fuel of a fossil fuel or the like, such as methane or LNG, and the reforming raw gas undergoes steam reforming to produce a reformed gas (fuel gas).
In the above steam reforming, water in correspondence with a quantity of water vapor used in the reforming reaction needs to be supplied. For this purpose, an approach where a required quantity of water is supplied from the outside has been adopted. Alternatively, a water collection approach by condensing the exhaust gas produced as a result of power generation in the fuel cell to achieve perfect circulation (water self-sustaining operation) of water needed for reforming has been drawing attention. In this regard, it is required to remove impurities from the condensed water. Therefore, water treatment equipment, e.g., an ion exchanger has been adopted.
For example, as shown in FIG. 13, a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2009-009732 (hereinafter referred to as conventional technique 1) includes a water tank 1a, and pure water refined by an unillustrated water treatment unit (ion exchange resin membrane) is supplied to the water tank 1a. An intake pipe 2a is provided at the bottom of the water tank 1a. The intake pipe 2a is connected to an air return mechanism 3a. The air return mechanism 3a is provided between the intake pipe 2a and a water pump 4a. The air return mechanism 3a includes an air separator 5a and an air return pipe 6a. 
If air bubbles are formed in the pure water supplied from the water tank 1a into the intake pipe 2a, before the air bubbles reach the water pump 4a, the air bubbles are guided into the air return pipe 6a, and returned to the water tank 1a. According to the disclosure, after the air bubbles are removed from the pure water, the pure water is supplied to the water pump 4a. 
Further, as shown in FIG. 14, a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2010-033917 (hereinafter referred to as conventional technique 2) includes a water purifier 1b. Water collected from the fuel cell system is guided into the water purifier 1b, and purified in the water purifier 1b to produce pure water. The water purifier 1b includes a container 4b filled with ion exchange resin 2b for producing pure water from the collected water and antibacterial agent 3b having a predetermined thickness above the ion exchange resin 2b in the direction of gravity. The water purifier 1b is an apparatus for producing pure water from the collected water supplied from a condenser (not shown). The pure water produced from the collected water in the water purifier 1b flows through a water passing pipe 5b, and the pure water is supplied to a water storage 6b. 
Further, as shown in FIG. 15, a fuel cell device disclosed in Japanese Laid-Open Patent Publication No. 2008-300058 (hereinafter referred to as conventional technique 3) includes a condensed water tank 1c for storing condensed water produced by heat exchange in a heat exchanger (not shown). An end of a condensed water supply pipe 2c is connected to a lower end of the condensed water tank 1c, and the condensed water supply pipe 2c is connected to the heat exchanger. A water tank 4c is connected to an upper end of the condensed water tank 1c through a tank coupling pipe 3c. The condensed water tank 1c contains, e.g., ion exchange resin 5c as means for treatment of the condensed water.
Further, as shown in FIG. 16, a fuel cell device disclosed in Japanese Laid-Open Patent Publication No. 2008-300059 (hereinafter referred to as conventional technique 4) includes a condensed water tank 1d. After the condensed water stored in the condensed water tank 1d is treated by condensed water treatment means (e.g., ion exchange resin) 2d, the condensed water flows through a tank coupling pipe 3d, and the condensed water is supplied to a water tank 4d. The water stored in the water tank 4d is supplied to a reformer (not shown) in correspondence with the quantity of water required in the reformer.
The condensed water tank 1d includes an upper partition member 5d and a lower partition member 6d. For example, the upper partition member 5d and the lower partition member 6d have mesh structure, or the upper partition member 5d and the lower partition member 6d are mesh-like members. The condensed water treatment means 2d is placed between the upper partition member 5d and the lower partition member 6d. 